Exposure device and method for compensating optical defects

ABSTRACT

An exposure device for exposing a projection of an electronically stored artwork pattern onto a substrate, in particular a printing plate  1,  with image processing electronics ( 2 ) that can store the image data, with a light modulator ( 7 ) that can be electronically controlled by the image processing electronics ( 2 ), in particular an LCD display ( 7 ) or a micro-mirror device, with an illuminating device ( 8, 9 ) for illuminating the light modulator ( 7 ), and with a projection lens ( 11 ) for projecting the light modulator ( 7 ) onto the substrate ( 1 ), is improved according to the invention in that the image processing electronics ( 2 ) include a compensation device ( 4, 5 ) to compensate for optical defects and/or tolerances in the beam path of the exposure device.

BACKGROUND OF THE INVENTION

The invention relates to an exposure device for exposing a projection ofan electronically stored artwork pattern onto a substrate, in particulara printing plate, with image processing electronics that can store theimage data, with a light modulator that can be electronically controlledby the image processing electronics, in particular an LCD display or amicro-mirror device, with an illuminating device for illuminating thelight modulator and with a projection lens for projecting the lightmodulator onto the substrate. The invention also relates to a method forcompensating for optical defects and/or tolerances in the beam path ofsuch an exposure device.

An exposure device of this kind, which is used to expose printing plateswith ultraviolet light, is known from DE 195 45 821 A1, for example. Inthis instance, the artwork pattern to be projected is broken down intopartial images by a computer and the partial images are cast one afterthe other onto an electronically controllable light modulator, forexample an LCD display through which radiation passes or a micro-mirrordevice. The exposure device is then gradually moved across the printingplate to be exposed, the light modulator being respectively triggeredwith the appropriate partial image. Micro-mirror devices, also referredto as digital mirror devices or DMD for short, are comprised of anarrangement of tiny mirrors with edge lengths of a few micrometers onthe surface of an electronic chip, in which each individual mirror canbe individually tilted by being electronically triggered so that anincident light beam can either be directed into a projection objectiveor directed past it, depending on the tilting angle of the mirror.

Due to optical defects and tolerances in the beam path of the exposuredevice, the projection exposed on the substrate to be exposed, inparticular a printing plate, deviates from the electronic artworkpattern. In particular, shadows are produced at the edges and in thecorners of the individual images.

SUMMARY OF THE INVENTION

The object of the current invention, therefore, is to disclose anexposure device of the type mentioned at the beginning as well as amethod, with which the optical defects and/or tolerances in the beampath of the exposure device can be compensated for.

The object is achieved according to the invention in the above-mentionedexposure device in that the image processing electronics include acompensation device to compensate for optical defects and/or tolerancesin the beam path of the exposure device. This electronic compensationdevice has the advantage that it can be produced simply andinexpensively, essentially in a software-based manner because imageprocessing electronics, preferably in the form of a computer, aregenerally already present and therefore do not entail any additionalexpenses.

A preferred embodiment of the invention provides that the compensationdevice can modulate the total quantity of incident light striking anypoint of the substrate to be exposed during the exposure process. Theaction of the exposure, i.e. the photochemical action finally producedon the substrate to be exposed, which in a photographic projection,corresponds to the luminosity of each respective point, essentiallydepends on the total quantity of incident light, which in the simplestcase of a constant light intensity, is the product of the intensity andthe exposure time. If each point of the substrate to be exposed can beindividually modulated, then this permits a virtually completecompensation down to all of the details of projection.

In one embodiment of the invention, during the exposure process, alocus-dependent intensity modulation of the stored artwork pattern canbe provided with an electronically stored gray mask. The gray mask iscomprised of a set of multiplication factors for each individual imagepoint. During the exposure, the intensity of each individual image pointthat the light modulator allows to pass through is increased ordecreased according to the stored multiplication factor and thereforethe luminosity differences unintentionally generated by defects and/ortolerances of the exposure- and/or projection lens can be compensatedfor. With the use of an LCD display as a light modulator, this can takeplace by virtue of the fact that the rotation angle of the circularlypolarized light is changed through appropriate control of the LCDdisplay.

In a micro-mirror device, an intensity modulation is not possible sincethe micromirrors can only be set to two fixed tilting positions so thatthe intensity of an individual point can only be switched to 100% on orcompletely off. A modulation of the total amount of incident lightstriking an individual point during the substrate exposure can only beexecuted by varying the “ON time” of each individual micro-mirror.Therefore, a modified embodiment of the invention provides that in orderto achieve a locus-dependent light quantity modulation, the ON times ofindividual pixels of the light modulator during the exposure process canbe varied in accordance with an electronically stored gray mask.

In order to simply generate a suitable gray mask, which compensates notonly for all defects of the exposure beam path, but also for theinfluence of the printing plate, its gamma curves, the developer used,and similar factors that can influence the luminosity distribution, amodification of the method according to the invention proposes that inorder to generate the electronically stored gray mask, a test exposureof a printing plate is executed, in which all pixels of the lightmodulator are triggered with an average gray value, and that after thedevelopment of the printing plate, the luminosity distribution producedon the printing plate is input into the image processing electronics,which generate the electronic gray mask based on this information,preferably by inverting the gray shades of the individual pixels.

The luminosity distribution of the developed test printing plate isinput into the image processing electronics simply by scanning theprinting plate with an electronic camera or an electronic scanner; thedata of the consequently digitized gray values are simply input into theimage processing electronics and are suitably converted therein to thedigital values of the gray mask and stored.

In the production of the electronic gray mask, the gradation of theprinting plate in connection with the respective exposure spectrum usedand the respective developer used can be precisely taken intoconsideration; all other influences on the luminosity distribution canbe “calculated out”. Such additional factors can, for example, be tracedback to the exposure intensity, the exposure time, the developmentduration, the composition, temperature, concentration, depletion (age),and impurities of the developer, or to the temperature, age,composition, or pre-treatment of the light-sensitive coating of theprinting plate. All conceivable influences on the luminositydistribution in the printing plate exposure can be compensated for bymeans of the method according to the invention through the electronicgray mask generated in a single calibration run.

Exemplary embodiments of the invention will be explained in detail belowin conjunction with the drawing.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of an exposure device according tothe invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The artwork pattern to be projected onto a printing plate 1 is stored inan image memory 3 in a computer that serves as the processingelectronics 2. An overlay file is stored in the overlay memory 4 and isused to generate an electronic gray mask. An image from the image memory3 or a partial image of the overall image is loaded into a compensator 5and is corrected pixel by pixel with the overlay data from the overlaymemory 4, i.e. the control data provided for controlling the lightintensity and/or the duration of the exposure of each individual pixelare modified through multiplication with the correction values of theoverlay file in accordance with the desired correction. As a rule, theirvalue is increased or decreased. Then, the corrected image or partialimage travels via the data line 6 to the light modulator, in this casean LCD display through which radiation passes, in whose focal plane theactual image is produced.

An illumination device for illuminating the light modulator 7 iscomprised of a light source 8 and a condenser 9. The light emanatingfrom the light source 8 is concentrated onto the light modulator 7 by acondenser 9. After the modulation in the light modulator 7, themodulated light strikes a mirror 10, which deflects a beam path downwardinto a projection lens 11. The projection lens 11 projects the focalplane of the light modulator 7 onto the printing plate 1.

In a modified embodiment, the mirror 10 can be replaced by amicro-mirror device, which is then used as the light modulator. In thiscase, the LCD display 7 can be eliminated.

The defects in the beam path of the exposure device, which are caused byoptical defects in the illumination device 8, 9, the projection lens 11,or also in other optical elements 7, 10, are compensated for by means ofthe electronic gray mask described above so that a largely error-freeand, in terms of luminosity reproduction, uniform projection of theelectronic artwork pattern is produced on the printing plate 1.

1. An exposure device for exposing a projection of an electronicallystored artwork pattern onto a substrate, in particular a printing plate(1), with image processing electronics (2) that can store the imagedata, with a light modulator (7) that can be electronically controlledby the image processing electronics (2), in particular an LCD display(7) or a micro-mirror device, with an illuminating device (8, 9) forilluminating the light modulator (7) and with a projection lens forprojecting the light modulator (7) onto the substrate (1), characterizedin that the image processing electronics (2) include a compensationdevice (4, 5) to compensate for optical defects and/or tolerances in thebeam path of the exposure device, and the compensation device (4, 5) canmodulate the total quantity of incident light striking any point of thesubstrate (1) to be exposed during the exposure process.
 2. An exposuredevice for exposing a projection of an electronically stored artworkpattern onto a substrate, in particular a printing plate (1), with imageprocessing electronics (2) that can store the image data, with a lightmodulator (7) that can be electronically controlled by the imageprocessing electronics (2), in particular an LCD display (7) or amicro-mirror device, with an illuminating device (8, 9) for illuminatingthe light modulator (7) and with a projection lens for projecting thelight modulator (7) onto the substrate (1), characterized in that theimage processing electronics (2) include a compensation device (4, 5) tocompensate for optical defects and/or tolerances in the beam path of theexposure device, and during the exposure process, a locus-dependentintensity modulation of the stored artwork pattern can be provided withan electronically stored gray mask.
 3. An exposure device for exposing aprojection of an electronically stored artwork pattern onto a substrate,in particular a printing plate (1), with image processing electronics(2) that can store the image data, with a light modulator (7) that canbe electronically controlled by the image processing electronics (2), inparticular an LCD display (7) or a micro-mirror device, with anilluminating device (8, 9) for illuminating the light modulator (7) andwith a projection lens for projecting the light modulator (7) onto thesubstrate (1), characterized in that the image processing electronics(2) include a compensation device (4, 5) to compensate for opticaldefects and/or tolerances in the beam path of the exposure device, andin order to achieve a locus-dependent light modulation, the ON times ofindividual pixels of the light modulator (7) during the exposure processcan be varied in accordance with an stored gray mask.
 4. A method forcompensating for optical defects and/or tolerances in the beam path ofan exposure device, in which a projection of an artwork pattern storedin a set of image processing electronics (2) is projected onto asubstrate, in particular a printing plate (1), in which a lightmodulator that is controlled by the image processing electronics (2), inparticular an LCD display (7) or a micro-mirror device, is illuminatedby means of an illuminating device (8, 9), and is projected onto thesubstrate (1) to be exposed by means of a projecting lens (11),characterized in that the total quantity of incident light striking anypoint of the substrate (1) during the exposure process is modulated insuch a way that the exposure differences on the substrate to be exposed,which are due to optical defects and/or tolerances in the beam path, arecompensated for.
 5. The method according to claim 4, characterized inthat during the exposure process, a locus-dependent intensity modulationof the stored artwork pattern takes place with an electronically storedgray mask.
 6. The method according to claim 4, characterized in that inorder to achieve a locus-dependent light quantity modulation, the ONtimes of individual pixels of the light modulator during the exposureprocess are varied in accordance with an electronically stored graymask.
 7. The method according to claim 4, characterized in that in orderto generate the electronically stored gray mask, a test exposure of aprinting plate (1) is executed, in which all pixels of the lightmodulator (7) are triggered with an average gray value, and that theluminosity distribution produced on the printing plate (1) after thedevelopment of the printing plate (1) is input into the image processingelectronics (2), which generate the electronic gray mask based on thisinformation, preferably by inverting the gray shades of the individualpixels.
 8. The method according to claim 7, characterized in that anelectronic camera or a scanner is used to input the luminositydistribution of the developed test printing plate (1).